Organosulfates are secondary organic aerosol (SOA) products that form from
reactions of volatile organic compounds (VOC), such as isoprene, in the
presence of sulfate that is primarily emitted by fossil fuel combustion. This
study examines the anthropogenic influence on biogenic organosulfate
formation at an urban site in Atlanta, Georgia (GA) in the southeastern
United States (US). Organosulfates were analyzed in fine particulate matter
(PM2.5) collected during August 2015 in Atlanta using hydrophilic
interaction liquid chromatography (HILIC), tandem mass spectrometry (MS/MS),
and high-resolution time-of-flight (ToF) mass spectrometry. By their MS/MS
response, 32 major organosulfate species were identified, selected species
were quantified, and other species were semi-quantified using surrogate
standards. Organosulfates accounted for 16.5 % of PM2.5 organic carbon
(OC). Isoprene-derived organosulfates were the most abundant, dominated by
methyltetrol sulfate which accounted for 12.6 % of PM2.5 OC.
Together, the isoprene-derived organosulfates accounted for the majority of
the isoprene-derived SOA that had been previously observed in Atlanta, but
had not been identified at the molecular level. Other major species included
seven monoterpene-derived organosulfates, five diesel and/or
biodiesel-derived organosulfates, and three new organosulfates that are also
expected to derive from isoprene. Organosulfate species and concentrations in
Atlanta were compared to those in a rural forested site in Centreville,
Alabama (AL) during summer 2013, which were also dominated by
isoprene-derived organosulfates. In Atlanta, isoprene-derived organosulfate
concentrations were 2–6 times higher and accounted for twice as much
OC. The greatest enhancement in concentration was observed for
2-methylglyceric acid sulfate whose formation is enhanced in the presence of
nitrogen oxides (NO and NO2; NOx) and is a tracer for isoprene
high-NOx SOA. The isoprene-derived organosulfates indicated a stronger
influence of NOx in Atlanta compared to Centreville. Overall, these
results suggest that SOA in the southeastern US can be reduced by controlling
NOx and SO2 emissions from fossil fuel combustion. This study gives
insights into the major organosulfate species that should be targets for
future measurements in urban environments and standard development.

Atlanta, Georgia (GA) is the principle city of the Atlanta metropolitan area (Atlanta–Sandy Springs–Roswell, GA), which is the ninth most populous metropolitan area in the
US as of 2017 with a population of 5.9 million (U.S. Census
Bureau, 2018). Here, OA accounts for 68 %–70 % of PM1 (fine particulate matter with aerodynamic diameter less than 1 µm) mass
(Rattanavaraha et al., 2017) and 71 % of PM2.5 mass during summer (Al-Naiema et al., 2019), the
majority of which is secondary in origin and mainly derived from biogenic VOC (Weber et al., 2007). For example, isoprene
dihydroxy epoxides (IEPOX) contributed 29 %–38 % of PM1 OA (Rattanavaraha et al., 2017; Budisulistiorini et al., 2016; Xu et al.,
2015a) and total isoprene-derived OA contributed to 27 % of PM2.5
organic carbon (OC) (Al-Naiema et al., 2019). The
diurnal variation of isoprene-derived OA in urban Atlanta, GA, was temporally consistent with
isoprene emissions from plants, suggesting that isoprene-derived OA forms locally
rather than being transported from surrounding forested sites
(Xu et al., 2015b). In Atlanta, sulfate is the second
largest component of fine PM and accounts for 15 % of PM2.5 (Al-Naiema et al.,
2019) and 17 %–21 % of PM1 mass (Rattanavaraha et al., 2017).
The aerosol acidity (average pH 1.4±0.7) and aerosol water
content (averaging 8.4±4.8µg m−3) in Atlanta also peak during summer (Rattanavaraha et al., 2017), similar
to other locations in the southeastern US
(Guo et al., 2015). In addition, previous studies
have demonstrated that the biogenic SOA formation in the southeastern US is
enhanced by sulfate, NOx, and O3, which mainly
come from fossil fuel combustion, particularly during summer when the biogenic
emissions are high (Goldstein et al., 2009; Gao et al., 2006; Xu et al.,
2015a; Carlton et al., 2010).

This study examines the anthropogenic influence on organosulfate formation
during summer at an urban site in Atlanta in the southeastern US. Our
specific objectives include the following: (1) identification and quantification of major
organosulfate species in Atlanta, GA, during August 2015 using hydrophilic
interaction liquid chromatography (HILIC), tandem mass spectrometry (MS/MS),
and high-resolution time-of-flight mass spectrometry (ToF-MS); (2) evaluation
of the factors that influence organosulfate formation via comparison of
observed species with SOA chamber experiments and correlations of
organosulfates with SOA tracers, other PM2.5 constituents, gas-phase
reactive species, and meteorological conditions; and (3) comparison of these
results with the major organosulfates identified and quantified in
Centreville, Alabama (AL) during summer 2013 (Hettiyadura et al., 2017, 2018) to better understand the extent to which
anthropogenic pollutants affect biogenic organosulfate formation across an
urban and rural pair in the southeastern US during summer. This study
provides insights into the composition, abundance, sources, and formation
pathways of organosulfates, which are useful as tracers for
anthropogenically influenced SOA.

2.2 PM2.5 sample collection

PM2.5 samples were collected in Atlanta, GA, from 29 July to 27 August 2015.
A medium volume sampler (3000B, URG Corp.) operating at a flow rate
of 90 L min−1 was used to collect PM2.5 on pre-baked (550 ∘C
for 18 h) quartz-fiber filters (90 mm,
Pallflex® Tissuquartz™, Pall life science).
The PM2.5 sampler was placed on the roof top of the School of Earth and
Atmospheric Sciences building at the Georgia Institute of Technology
(33∘46′44.2′′ N, 84∘23′46.2′′ W; height ∼30–40 m). A detailed description of the sampling site is provided by
Verma et al. (2014). Samples were collected daily from 13:30 to
12:30 the next day (local time). One filter blank was collected for every
five PM2.5 samples. Samples from 29 July, 3, 11, and 19 August were
not analyzed for organosulfates, as the filters were used for a different
purpose. The collected samples were placed in aluminium-lined (pre-baked at
550 ∘C for 18 h) petri dishes, sealed with Teflon tape, and
stored at −20∘C until extracted.

2.3 Extraction of organosulfates

Organosulfates were extracted according to the method described in
Hettiyadura et al. (2015) that has been demonstrated to efficiently recover
83 %–121 % of organosulfates with aliphatic, aromatic, carbonyl, hydroxyl,
and carboxyl acid groups. Briefly, subsamples of filters (averaging
∼ 3 cm2) were extracted with 10.0 mL of acetonitrile and
ultra-pure water (95:5, by volume) for 20 min by ultra-sonication (5510,
Branson). The sample extracts were filtered using polypropylene membrane
syringe filter discs (0.45 µm pore size, Puradisc™ 25 PP,
Whatman®). The extracts were evaporated to dryness under
ultra-high purity nitrogen gas at 50 ∘C (Turbovap®
LV, Caliper Life Sciences, Reacti-Therm III TS 18824, and Reacti-Vap I 18825,
Thermo Scientific). Dried extracts were reconstituted in 600 µL
of acetonitrile and ultra-pure water (95:5 by volume).

2.4 Quantification of organosulfates

Organosulfates were quantified using HILIC and tandem mass spectrometry (MS/MS) in negative (-) ion mode, using an
ultra-performance liquid chromatography system (UPLC, ACQUITY UPLC H-Class,
Waters) coupled with a triple quadrupole (TQ) mass spectrometer (AQCUITY,
Waters) and an electrospray ionization (ESI) source. The separation of organosulfates was performed on an
ethylene-bridged hybrid amide column using an acetonitrile-rich mobile phase
(acetonitrile and ultra-pure water; 95:5) and an aqueous mobile phase
(ultra-pure water; 100 %). Both mobile phases were buffered at pH 9 with
10 mM ammonium acetate and ammonium hydroxide. Organosulfates were eluted
using a stepwise gradient as described in Hettiyadura
et al. (2015). Briefly, the acetonitrile-rich mobile phase was held at
100 % from 0 to 2 min, and then decreased to 85 % from 2 to 4 min
and held constant at 85 % until 11 min. Targeted analysis was
performed in multiple reaction monitoring mode. Hydroxyacetone sulfate and
glycolic acid sulfate were quantified using authentic standards. Lactic acid
sulfate and methyltetrol sulfate were quantified using their response
factors determined previously using authentic standards. Notably, these
prior experiments had response factors (determined as the slope of the
calibration curve) for glycolic acid sulfate and hydroxyacetone sulfate that
were within 10 % of the current experiments, indicating that instrument
performance and ionization were consistent within 10 %. The optimized
ESI(-)-MS/MS conditions used for each of these organosulfates are given in
Hettiyadura et al. (2015, 2018), respectively.

Semi-quantitation of other organosulfates was based upon the MS/MS response
of authentic standards and matched to the sulfur-containing fragment ions
observed. For semi-quantitation of organosulfates that fragmented to the
bisulfate anion (m∕z 97, Fig. 1a), one of three surrogate standards were used: for m∕z 211, 213, and 260 the response factor of methyltetrol sulfate was used; for other organosulfates eluting prior to
4 min hydroxyacetone sulfate was used, and for those retaining more
than 4 min glycolic acid sulfate was used. For the semi-quantitation
of organosulfates that fragmented only to the sulfate radical anion (m∕z 96,
Fig. 1b), methyl sulfate was used. For organosulfates with m∕z 137, 139, and 296
that fragmented to the sulfite radical anion (m∕z 80, Fig. 1d) hydroxyacetone
sulfate was used. The cone voltage and collision energy used for the
organosulfates that were semi-quantified using surrogate standards were the same
as the ESI(-)-MS conditions used for corresponding precursor ion scans
(given in Sect. 2.5.1). The uncertainty of the organosulfate concentrations
was calculated accounting for relative errors in air volume, extraction
efficiency, and instrumental analysis according to the method described in
Hettiyadura et al. (2017). The relative error
in the instrument analysis was propagated using the limit of detection and
the relative standard deviation for each organosulfate standard given in
Hettiyadura et al. (2015). For methyltetrol sulfate
and the other organosulfates that did not have authentic standards, the
analytical uncertainty was estimated as 30 % of their concentration values
(Hettiyadura et al., 2018). This uncertainty does not
account for any bias introduced by the use of a surrogate standard, which
can only be evaluated by using an authentic standard. All data were acquired
and analyzed using MassLynx and QuanLynx softwares (Waters Inc., version 4.1).

Figure 1Precursors of (a) bisulfate ion (m∕z 97), (b) sulfate ion
radical (m∕z 96), (c) bisulfite ion (m∕z 81), and (d) sulfite ion radical (m∕z 80)
identified from a sample collected on 30 July 2015 in Atlanta. Blue
indicates nominal m∕z of the major organosulfate species that were identified
from the precursor m∕z 97 scan. Red indicates nominal m∕z of the major
organosulfate species that were identified from the precursor m∕z 96, 81, and 80
scans.

2.5 Qualitative analysis of organosulfates

2.5.1 Precursor ion scans

Sample analysis was performed on the UPLC-TQ in precursor ion mode as
described in Hettiyadura et al. (2017). Briefly, a respective cone voltage and
collision energy of 28 V and 16 eV were used for the m∕z 97 precursor ion
scan, whereas a respective cone voltage and collision energy 42 V and 20 eV were used for the m∕z 96 precursor ion scan. In addition,
precursor ion scans of m∕z 81 (bisulfite anion) and m∕z 80 were used to identify
organosulfates that did not fragment into m∕z 97 or 96, for which a cone
voltage of 34 V and a collision energy of 18 eV were used. A mass range of
100–400 Da was used in all precursor ion scans. The data were acquired and
analyzed using MassLynx and QuanLynx software packages (Waters Inc., version 4.1).

2.5.2 Chemical characterization and structure elucidation

PM extracts were also analyzed by a UPLC-ToF mass spectrometer (Bruker
Daltonics MicrOTOF) to determine the elemental composition and structural
information of the major sulfur-containing species. The ESI(-) conditions
included a capillary voltage of 2.6 kV, a cone voltage of 30 V, and a
desolvation gas flow rate of 600 L h−1. Other ESI(-)-MS conditions used
were the same as in Hettiyadura et al. (2015). Data
were collected in a mass range from 100 to 400 Da. A peptide, Val-Tyr-Val (m∕z 378.2029,
Sigma-Aldrich), was used as the lock mass to correct for any
instrument drift. Molecular formulas were assigned considering both odd and
even electron states, C1−25, H0−50, O3−20, S1−2,
N0−5, and a maximum error of 10 mDa. The data were acquired and
analyzed using MassLynx software (Waters Inc., version 4.1) and an elemental
composition tool (Waters Inc., version 4.0).

2.6 Collocated measurements

Percent contributions of organosulfates to PM2.5 OC were compared to
determine the relative abundances of the major organosulfates in Atlanta and
Centreville. OC in the PM2.5 samples was measured on 1 cm2 filter
punches using a thermal–optical analyzer (Sunset laboratory) according to
Schauer et al. (2003). Filter-based
measurements of other PM2.5 components, gas-phase measurements, and
meteorological conditions were used in correlation analysis to provide
insight to precursors and formation pathways of organosulfates. Isoprene SOA
tracers (2-methylthreitol, 2-methylerythritol, 2-methylglyceric acid,
cis-2-methyl-1,3,4-trihydroxy-1-butadiene,
3-methyl-2,3,4-trihydroxy-1-butene, and
trans-2-methyl-1,3,4-trihydroxy-1-butene), cis-pinonic acid, β-caryophyllinic acid,
meso-erythritol, 2,3-dihydroxy-4-oxopentanoic acid,
aromatic dicarboxylic acids (phthalic acid, terephthalic acid, isophthalic
acid, and 4-methylphthalic acid), and mononitroaromatic compounds
(4-nitrophenol, 2-methyl-4-nitrophenol, 4-methyl-2-nitrophenol,
4-nitrocatechol, 3-methyl-6-nitrocatechol, and 3-methyl-5-nitrocatechol)
were measured by gas chromatography (GC)-MS according to the methods
described in Al-Naiema and Stone (2017). Sulfate was
measured by ion chromatography following Jayarathne et al. (2014). The
hourly based measurements of O3, NOx (nitrogen oxides such as, NO
and NO2), and solar radiation were obtained from the Southeastern
Aerosol and Research Characterization network monitoring site at Jefferson
Street (JST) located 2 km west of the sampling site and were averaged across
the sample collection time. Detailed descriptions of their quantification
methods are described in Hansen et al. (2003).

3.1 Quantitative analysis of organosulfates

Quantitative information about the organosulfates observed in Atlanta is
summarized in Table 1, with time series of selected species shown in Fig. 2.
Methyltetrol sulfate is the most abundant quantified organosulfate,
contributing 12.6 % of PM2.5 OC, followed by m∕z 211 (0.93 %), 213
(0.80 %), glycolic acid sulfate (0.24 %), 2-methylglyceric acid sulfate
(0.32 %), and lactic acid sulfate (0.20 %) (Table 2). The remaining 26
organosulfates were estimated to contribute 1 % of PM2.5 OC.
Altogether, the 32 measured organosulfates in Table 1 account for 16.5 %
of PM2.5 OC. These results indicate that organosulfates in Atlanta
during August 2015 were dominated by methyltetrol sulfate, with minor
contributions from many other organosulfate species derived from isoprene,
monoterpenes, and anthropogenic sources.

Table 1The major organosulfates identified using HILIC-TQ in
daily PM2.5 samples collected from Atlanta, GA, in August 2015,
indicating nominal mass-to-charge ratio (m∕z), chemical formula, and
monoisotopic mass determined from HILIC-ToF, proposed structure
(with a star indicating many isomers, although only one is shown),
potential VOC precursors, and their average ambient concentrations with one
standard deviation (SD). For these organosulfates the median and the maximum
error in the observed mass is 1.7 and 7.5 mDa, respectively. Organosulfates
are ordered in the table from greatest to lowest abundance.

3.2 Qualitative analysis of major organosulfates

Organosulfates were identified by precursors to m∕z 97 (HSO4-), 96
(SO4-•), 81 (HSO3-), and 80 (SO3-•) in three
PM2.5 samples collected on 30–31 July and 1 August 2015. Results
were similar for all three samples; therefore, the results obtained only for
the 30 July sample are shown in Fig. 1. Major organosulfur compounds were
defined in one of the two following ways: (1) as having a minimum relative intensity in the
MS/MS spectra (≥1.0 % for precursors to m∕z 97, >12 % for
m∕z 96, >5 % for m∕z 81, and >3 % for m∕z 80 in any of
the three samples) or (2) by retaining more than 4 min. Despite the
observation that organosulfates eluting after 4 min often have higher
concentrations than early eluting species, their MS response is observed to
be lower because of the increased water content of the mobile phase as water
does not desolvate as efficiently as acetonitrile in the ESI source
(Hettiyadura et al., 2017). The absolute MS
signal for precursors to m∕z 97 was 52, 10, and 8 times greater than MS signals for precursors to m∕z 96,
81, and 80, respectively; however, due to differing ionization efficiencies
and stabilities among these fragment ions, the strength of the MS signal is
not indicative of the relative concentrations of species that form these
fragments. Table 1 summarizes the major organosulfates' elemental
composition, monoisotopic mass, proposed or known structures, and precursor
gases. Of the major organosulfates, 26 of the 32 consisted of C, H, O, and
S, while 6 of 32 consisted of C, H, O, S, and N. Structures were proposed
based on elemental composition, double bond equivalence (DBE), retention
time, and prior studies.

Table 2Comparison of organosulfates quantified or semi-quantified
in Centreville, AL, from 13 June to 13 July 2013 and in Atlanta, GA, in
August 2015. Standard deviations are given in parenthesis.

a Published in Hettiyadura et al. (2018),
b quantified against authentic standards or response factors detected in
a previous experiment, c semi-quantified against hydroxyacetone
sulfate, d semi-quantified against glycolic acid sulfate, and e semi-quantified against methyltetrol sulfates or using its response
factor.

3.3 Isoprene-derived organosulfates

The strongest organosulfate signals observed in m∕z 97, 80, and 81 precursor
ion scans are associated with isoprene (Fig. 1, Table 1). Methyltetrol
sulfate (m∕z 215), the most abundant organosulfate observed, is produced from
the acid catalyzed nucleophilic addition of sulfate to IEPOX ring
(Surratt et al., 2010). Organosulfates with m∕z 211
(hydroxymethyl-tetrahydrofuranone sulfates) and 213
(dihydroxymethyl-tetrahydrofuranyl sulfates), of the next-highest
abundance, have been observed during the photooxidation of isoprene
(Surratt et al., 2008) and are suggested to
derive from the oxidation of primary alcohols in methyltetrol sulfates
(Hettiyadura et al., 2015). In addition, 14 other
major organosulfates identified are known to derive from isoprene and
isoprene oxidation products (Table 1). Many of these organosulfates have
also been identified as SOA products from diesel and biodiesel fuel
emissions (e.g., 2-methylglyceric acid sulfate, lactic acid sulfate,
hydroxyacetone sulfate, m∕z 167, 183, 197, 211, 213, 237, 239, and 253)
(Blair et al., 2017), monoterpenes (m∕z 239 and 253)
(Surratt et al., 2008), and/or MBO (199;
C5H11SO6-) (Zhang et al., 2012). However, their
moderate to strong correlations with methyltetrol sulfate (Table S1 in the Supplement) and
2-methyltetrols (Table S2) suggest that they are mainly derived from
isoprene.

Among the major organosulfate signals are those associated with isoprene
oxidation under high-NOx conditions such as 2-methylglyceric acid
sulfate, m∕z 260 and 274. 2-Methylglyceric acid sulfate is a tracer for
isoprene high-NOx SOA that is formed by the acid-catalyzed nucleophilic
addition of sulfate to methacrylic acid epoxide (MAE) and/or
hydroxymethyl-methyl-α-lactone (HMML) (Lin et
al., 2013). The organosulfate with m∕z 260 is a nitrooxy organosulfate that
derives from the photooxidation of isoprene under high-NOx conditions
(Surratt et al., 2008; Gómez-González et al., 2008). Two isomers
of m∕z 260 were identified in this study, while up to four isomers of m∕z 260 were
reported in Centreville (Surratt et al., 2008).
The m∕z 260 also correlated moderately with methyltetrol sulfate (r=0.539,
p value =0.005, Table S1), supporting its formation from isoprene. The
organosulfate with m∕z 274 is also a nitrooxy organosulfate that is derived
from isoprene photooxidation under high-NOx conditions (Nestorowicz
et al., 2018). The organosulfate with m∕z 274 has multiple isomers, whereas only
the two isomers retaining greater than 4 min are considered to be major ones
as described in Sect. 3.2 (Fig. 3o). Their longer retention times (5.6 and
5.8 min), three additional oxygen atoms, and one unit of unsaturation
suggest the presence of a carboxylic acid functional group and a hydroxyl
group. Plausible structures for these two organosulfates are diastereomers
of 2-carboxy-3-hydroxy-4-(nitrooxy)butan-2-yl sulfate (Table 1), which could
form by the oxidation of a primary hydroxyl group in
1,3-dihydroxy-2-methyl-4-(nitrooxy)butan-2-yl sulfate (an isomer of m∕z 260,
C5H10SO9-, proposed by Darer et al., 2011) to a carboxylic acid. The strong correlation of these two signals at
m∕z 274 with the less-oxidized isoprene nitrooxy organosulfate (m∕z 260)
(r=0.860, p value <0.001, Table S1) supports this prediction.
Overall, these results indicate that isoprene is the major precursor of the
most abundant organosulfates in this study.

Figure 3Extracted chromatograms of 19 major organosulfate species
obtained from a PM2.5 sample collected in Atlanta using HILIC-ToF. Extracted chromatograms of the remaining 13 major
organosulfate species are shown in
Hettiyadura et al. (2017) for a PM2.5 sample collected in Centreville. MS data, structures, and VOC precursors
of these organosulfates are given in Table 1.

Isoprene-derived organosulfates explain a significant fraction of
isoprene-derived organic aerosol observed in Atlanta that had not previously
been identified on a molecular level. By factor analysis of aerosol chemical
speciation data (using the multilinear engine – ME-2), IEPOX-derived OA was
estimated to account for 29 % (3.3 µg m−3) of PM1 OA at the nearby JST monitoring site in
summer 2014, whereas the IEPOX-OA tracers measured in PM2.5
(2-methyltetrols, C5-alkene triols, and 3-methyl-hydrofuran-3,4-diols)
accounted for 3 % of PM1 OA (Rattanavaraha
et al., 2017), assuming negligible differences between PM1 and
PM2.5. The remaining IEPOX-derived OA corresponded to 10 %–18 % of
PM1 OC (considering an OM:OC ratio of 2.05±0.57)
(Xu et al., 2017), and is comparable to the contribution
of isoprene-derived organosulfates to PM2.5 OC in this study
(15.7 %). Additionally, the isoprene-derived organosulfates observed in
this study account for more than half of the PM2.5 secondary organic
carbon coming from isoprene, which is estimated as 27 % following the SOA
tracer method (Al-Naiema et al., 2019; Kleindienst et al.,
2007). These results indicate that more than half of the isoprene-derived OA
in Atlanta during summer is comprised by organosulfates, mainly methyltetrol
sulfate.

3.4 Monoterpene-derived organosulfates

Seven of the thirty-two major organosulfates identified in Atlanta (Table 1) were
previously detected among the SOA produced from monoterpenes in the presence
of NOx and acidic sulfate seed aerosols
(Surratt et al., 2008). Of these,
nitrooxy organosulfates at m∕z 342, 294, and 296 are derived from monoterpenes
either by photooxidation in the presence of NOx or from nitrate
radical-initiated oxidation (Surratt et al., 2008; Iinuma et al., 2007).
The estimated contribution of these seven monoterpene-derived organosulfates
is 0.5 % of PM2.5 OC. However, the accuracy of this estimate is
limited by the lack of authentic standards for monoterpene organosulfates
and the large differences in molecular structure between the monoterpene
organosulfates and the standards utilized in this study. The absence of
significant correlations among nitrooxy organosulfates with other
organosulfates (Table S1) and biogenic SOA tracers that predominantly derive
from photooxidation reactions (Table S2) suggest that these
nitrooxy organosulfates likely formed by nitrate radical-initiated
oxidation. Organosulfates with m∕z 223, 279, and 281 have been identified as
SOA products of α-pinene, as well as from other monoterpenes (m∕z 279
and 281), in the presence of NOx and highly acidic sulfate seed aerosol
(Surratt et al., 2008). The organosulfate with
m∕z 251 has been identified in SOA from the photooxidation of β-caryophyllene
(a sesquiterpene) and limonene (a monoterpene) in the
presence of NOx and sulfate seed aerosols (Chan et al., 2011;
Surratt et al., 2008). These species did not correlate with β-caryophyllinic
acid (Table S2), a SOA tracer for β-caryophyllene
formed under high-NOx conditions (Jaoui et al., 2007), suggesting
that m∕z 251 mainly forms from monoterpenes. Organosulfates with the same
m∕z were also detected among the organosulfates generated from diesel and
biodiesel fuel emissions (Blair et al., 2017) and
photooxidation of n-alkanes such as decaline (m∕z 281) and cyclodecane (m∕z 279 and
281) (Riva et al., 2016b), but these species are expected to be biogenic
in nature due to the dominance of biogenic VOC in Atlanta during summer
(Geron et al., 1995; Al-Naiema et al., 2019; Rattanavaraha et
al., 2017).

3.5 Organosulfates derived from anthropogenic sources

Five organosulfates that were previously reported only in photooxidation of
diesel and/or biodiesel fuel in the presence of SO2 were identified
among the thirty-two major organosulfates. These include m∕z 137 and 151 that were
generated from diesel fuel emissions and m∕z 195, 209, and 265 that were
generated from both diesel and biodiesel emissions
(Blair et al., 2017). The organosulfate with m∕z 265
corresponds to dodecyl sulfate, a widely used surfactant in detergents that
can also come from wastewater treatment plants
(Hettiyadura et al., 2017). The
concentrations of m∕z 209 and 195 are at least 3 times higher compared with other
organosulfates derived from diesel and/or biodiesel emissions in this study
(Table 1). These organosulfates (m∕z 209 and 195) were also detected with a
high abundance in urban Shanghai and Los Angeles (Tao et al.,
2014). The organosulfates with m∕z 209 and 195 are homologs, differing by one
methylene. Both compounds have two units of unsaturation and two additional
oxygen atoms. Further, their retention times (Fig. 3f and h), which were
less than 1 min, suggest that they do not contain a carboxylic acid
group, but may contain two carbonyl groups. Additional work is required to
determine the position of carbonyl and sulfate groups in these compounds. As
m∕z 209 and 195 are highly abundant in other urban locations and are only known
to derive from diesel and/or biodiesel fuel, they may be useful as tracers
for SOA derived from diesel and biodiesel emissions.

3.6 Aromatic organosulfates

Aromatic sulfur-containing compounds were not detected among the major
organosulfate species (Table 1), although some were observed by HILIC-ToF. Two
sulfur-containing compounds had large DBEs indicating aromatic groups: m∕z 185
(tR 1.06 min, C7H5SO4-, DBE 5.5, error 3.7 mDa) and
201 (tR 7.56 and 8.17 min, C7H5SO5-, DBE 5.5, error
3.5 mDa). The MS data matched the molecular formula reported by
Riva et al. (2015), who detected m∕z 185 in naphthalene and
2-methylnaphthalene photooxidation experiments and identified it as
formylbenzenesulfonate by MS fragmentation. Riva et al. (2015) also reported
m∕z 201 in SOA generated by the photooxidation of 2-methylnaphthalene and
identified it as 4-sulfobenzoic acid using an authentic standard. In the
Atlanta PM2.5, two isomers of m∕z 201, likely conformational isomers of
4-sulfobenzoic acid, are observed. The presence of a carboxylic acid group
in m∕z 201 is evident by the retention time >7min in the HILIC
method (Hettiyadura et al., 2015). None of the aromatic organosulfates
reported in Staudt et al. (2014) (phenyl sulfates and
benzyl sulfates) were detected in HILIC-ToF. This may be due to the lower
retention times and higher detection limits for aromatic organosulfates in
HILIC compared to reversed-phase LC (Hettiyadura et
al., 2015). These results suggest that aromatic organosulfates have low
PM2.5 concentrations in comparison to biogenic organosulfates in
Atlanta during the summertime.

3.7 Additional organosulfates observed in ambient aerosol

Three organosulfates that have not been previously reported in laboratory
smog chamber experiments were detected among the major organosulfate
signals: m∕z 155 (C3H7SO4-), 165, and 242. These signals
were previously detected in PM2.5 in Centreville, AL
(Hettiyadura et al., 2017), while new
insights to their possible precursors and structures are gained here. The
species with m∕z 155 was previously identified as a mono-hydroxy propyl sulfate
(Hettiyadura et al., 2017); in Atlanta, it
correlated with most of the isoprene-derived organosulfates (Table S1),
suggesting that it was derived from isoprene.

The organosulfate at m∕z 165 has an elemental composition of
C4H5SO5-, indicating the presence of sulfate, an
additional oxygenated functional group, and two DBEs. The ToF chromatograms
(Fig. 3d) indicate two isomers of m∕z 165 that eluted in less than 2 min. While
both isomers fragmented to m∕z 80, only the first isomer fragmented into m∕z 96,
which was quantified. Its elemental composition and DBE suggest a
dihydrofuran ring structure (Table 1). The strong correlations of m∕z 165 with
methyltetrol sulfate (r=0.720, p value <0.001; Table S1) and
2-methyltetrols (r=0.670 and 0.768, p value <0.001; Table S2)
suggest that it is also derived from isoprene.

The organosulfate at m∕z 242 has an elemental composition of
C5H8NSO8-, indicating the presence of sulfate, nitrooxy,
an additional oxygenated functional group, and two DBEs. Its short retention
time of 0.5 min (Fig. 3k) suggests that it contains a carbonyl group as
organosulfates with hydroxyl and carboxylate groups retain more than 1 and 4 min,
respectively (Hettiyadura et al., 2015,
2017). A possible formation pathway for this nitrooxy organosulfate can be
loss of a water molecule from 2,3-dihydroxy-3-methyl-4-(nitrooxy)butyl
sulfate (an isomer of m∕z 260, C5H10SO9-, proposed by
Gómez-González et al., 2008) forming an enol that tautomerizes
to a carbonyl forming 3-methyl-4-(nitrooxy)-2-oxobutyl sulfate (Table 1).
Only a few atmospherically relevant isoprene-derived nitrooxy organosulfates
have been identified in previous studies. These include m∕z 244, 260, 274, and
305 that are derived from isoprene photooxidation under high-NOx
conditions (Surratt et al., 2008; Gómez-González et al., 2012).
It is expected that m∕z 242 is an additional nitrooxy organosulfate that has
not been previously identified in isoprene photooxidation experiments. As
m∕z 242 nitrooxy organosulfate is expected to derive from m∕z 260, it may provide
insight to the atmospheric aging of isoprene-derived SOA, although further
evaluation is needed.

3.8 Comparison of major organosulfates in Atlanta and Centreville

To better understand the extent to which anthropogenic pollutants influenced
biogenic SOA formation in urban Atlanta during August 2015, the
concentrations of the major organosulfates were compared to those measured
in rural Centreville, AL, during summer 2013 analyzed by similar methodology
(Hettiyadura et al., 2017). Although the major
organosulfates identified at both sites were similar and mainly derived from
isoprene, their concentrations were 2–6 times higher in Atlanta than
in Centreville, with the greatest enhancement obtained for 2-methylglyceric acid
sulfate (Table 2). As the absolute concentrations of these organosulfates
vary with time due to changes in meteorology, which affect isoprene
emissions, transport, and mixing of biogenic and anthropogenic pollutants,
their relative contributions to PM2.5 OC were compared across the two
sites (Table 2). In total, 12 organosulfates quantified or semi-quantified
in both studies contributed 7 % of PM2.5 OC in Centreville, and
16 % in Atlanta. These 12 organosulfates accounted for 95 % of the total
organosulfate mass in Atlanta and 58 %–78 % of the total bisulfate ion
signal in Centreville (Hettiyadura et al.,
2017), indicating that these were the dominant species at both sites.
Similarly, the IEPOX-OA in Atlanta during August 2012 (31 % of PM1
OA) was ∼ 2 times greater than IEPOX-OA in Centreville in
summer 2013 (18 % of PM1 OA) (Xu et al., 2015a, b).
Overall, these results suggest isoprene SOA is 2 times higher in Atlanta
compared with Centreville during summer.

Correlations of major organosulfate species were examined at both the Atlanta
and Centreville sites to gain insight into their sources and formation
pathways. Organosulfates at both sites show moderate to strong correlations
with isoprene, isoprene oxidation products, and/or isoprene SOA tracers
(Table S2; Table S6 in Hettiyadura et al., 2018),
supporting that they mainly derive from isoprene. The correlations of
inorganic sulfate with most of the organosulfates were weak or negligible in
Atlanta (Table S4), but were moderate to strong in Centreville (r=0.5–0.8) (Table S6 in Hettiyadura et al., 2018). This is
likely due to the consistently high levels of sulfate observed in urban
Atlanta (ranging from 0.82 to 3.24 µg m−3 and averaging 1.70±0.58µg m−3) compared with more variable sulfate
concentrations in rural Centreville (ranging from 0.42 to 4.17 µg m−3 and averaging 1.78±0.81µg m−3) (Hettiyadura et al., 2017). Overall these
results suggest isoprene and sulfate are important factors influencing the
organosulfate formation in both urban Atlanta and rural Centreville.

Isoprene-derived organosulfates indicated a stronger influence of NOx
on their formation in Atlanta compared to Centreville. NOx influence
is evident by the elevated levels of high-NOx isoprene oxidation
products such as 2-methylglyceric acid sulfate, which was 6 times higher
in Atlanta than in Centreville, and the isoprene-derived nitrooxy
organosulfate at m∕z 260 being the eighth strongest organosulfate signal in
Atlanta. These results are consistent with the average NOx
concentration in urban Atlanta in August 2015 (10.5 ppb) that was 15 times
greater than the average NOx concentration in rural Centreville during
summer 2013 (0.7 ppb) (SOAS, 2013). Methyltetrol sulfate, the most
abundant organosulfate at both sites, is thus expected to derive from
low-NOx oxidation pathway in Centreville as described in
Surratt et al. (2010) and by high-NOx oxidation
pathway in Atlanta as described in Jacobs et al. (2014). The moderate
and strong correlations obtained for isoprene-derived organosulfates with
high-NOx SOA products (Table S3) such as meso-erythritol
(Angove et al., 2006) and nitroaromatic compounds
(Al-Naiema and Stone, 2017), as well as with ozone (Table S4) that is formed by
the photochemical reactions of NOx and VOC
(Blanchard et al., 2014), also support that NOx plays
a key role in isoprene-derived organosulfate formation in Atlanta. However,
organosulfate formation from ozonolysis (Riva et al.,
2016a) cannot be ruled out. While these findings are consistent with other
studies that indicate a substantial influence of anthropogenic SO2 and
NOx on biogenic SOA formation in the southeastern US during summer
(Rattanavaraha et al., 2016; Xu et al., 2015a), this study provides
evidence for a greater influence of NOx on isoprene-SOA formation in
urban Atlanta, GA, compared to rural Centreville, AL, in summer.

This study provides insights to the major organosulfate species that should
be targets for future measurements and standard synthesis. The three most
abundant organosulfates measured in both Atlanta and Centreville include
methyltetrol sulfate, m∕z 211, and 213. Of these, only a standard for
methyltetrol sulfate was previously synthesized (Budisulistiorini et al.,
2015; Bondy et al., 2018). Given the ubiquity and high abundance of m∕z 211 and 213 in
the southeastern US and other locations (Hettiyadura et al., 2017;
Spolnik et al., 2018), they should be the next highest priorities for
authentic standard development. The m∕z 211 and 213 also have multiple isomers
as described by Hettiyadura et al. (2015) and
Spolnik et al. (2018). Further, this study reveals isoprene-derived
organosulfates such as 2-methylglyceric acid sulfate and m∕z 260 are useful in
distinguishing SOA formed under high-NOx conditions in urban
environments.

While isoprene was the major precursor to organosulfates at both Atlanta and
Centreville, the comparison of these two datasets reveals different
anthropogenic influences on biogenic SOA formation (Sect. 3.8). In
particular, NOx had a stronger influence on organosulfate formation in
Atlanta, and sulfate had a stronger influence on organosulfate formation
in Centreville. Future studies should focus on comparing the major
organosulfate species in other urban and rural locations in the southeastern
US to determine if these trends are ubiquitous across urban–rural landscapes
and to better understand the anthropogenic influences on biogenic SOA
formation. While high levels of isoprene-derived organosulfates detected in
the southeastern US during summer coincide with high isoprene emissions from
plants, high levels of aromatic organosulfates and nitrooxy organosulfates
detected in fall and winter coincide with high levels of biomass burning
(Ma et al., 2014; He et al., 2014). Thus, longer-term measurements of
organosulfates spanning an annual cycle are needed to further evaluate the
sources and concentrations of organosulfates in the atmosphere.

The authors would like to thank Emily Geddes, Kaitlin Richards, and Tim Humphry at
the Truman State University for synthesizing standards of hydroxyacetone
sulfate and glycolic acid sulfate; Sean Staudt at the University of Wisconsin,
Madison for synthesizing the lactic acid sulfate standard; Jason D. Surratt, Avram Gold and Zhenfa Zhang
at the University of North Carolina at Chapel Hill for
providing the methyltetrol sulfate standard; Josh Kettler and Carter Madler for
their assistance in sample preparation and analysis; Lynn Teesch and Vic Parcell for
their assistance in the University of Iowa High Resolution Mass
Spectrometry Facility (HRMSF); and Rodney J. Weber for assistance with sample
collection. This research was supported by the National Science Foundation
AGS grant number 1405014.

Kristensen, K. and Glasius, M.: Organosulfates and oxidation products from
biogenic hydrocarbons in fine aerosols from a forest in North West Europe
during spring, Atmos. Environ., 45, 4546–4556, https://doi.org/10.1016/j.atmosenv.2011.05.063, 2011.

This study examines anthropogenic influences on secondary organic aerosol at an urban site in Atlanta, Georgia. Organosulfates accounted for 16.5 % of PM2.5 organic carbon and were mostly derived from isoprene. In contrast to a rural forested site, Atlanta's isoprene-derived organosulfate concentrations were 2–6 times higher and accounted for twice as much organic carbon. Insights are provided as to which organosulfates should be measured in future studies and targeted for standard development.

This study examines anthropogenic influences on secondary organic aerosol at an urban site in...